(1) Field of the Invention
The present invention relates to a semiconductor lithography technique and particular to an X-ray mask suitable for lithography using a soft X-ray and a method for producing it.
(2) Prior Art
Silicon nitride having a stoichiometric atomic composition, namely, SiN.sub.1.33, has an internal tensile stress as large as about 2.times.10.sup.9 N/m.sup.2 and, when a membrane is produced therefrom by deposition and if the thickness exceeds a certain value, the breaking strength is exceeded causing cracks and thus it is not preferred to produce SiN.sub.1.33 membranes. Therefore, the membrane of silicon nitride SiNx which is a material capable of transmitting a soft X-ray as an X-ray mask substrate material has been prepared by adjusting an NH.sub.3 /SiH.sub.4 ratio of reaction gas in the chemical vapor deposition in order to reduce the internal stress as disclosed in Japanese Patent Kokai No. 54-53965. The thus prepared membrane of silicon nitride SiNx had a desired atomic composition and a reduced internal tensile stress and thus the effective tensile strength of the silicon nitride SiNx membranes per se was increased. Such silicon nitride SiNx has been used as an X-ray mask substrate material.
Since generally X-ray lithography is a system for positioning an X-ray transmission type mask close to a silicon wafer or the like and transferring a mask pattern in a ratio of 1:1, a high dimensional precision is required for the X-ray mask per se. An X-ray mask comprises a mask substrate composed of a thin membrane of a light element material which can easily transmit a soft X-ray and a pattern of a heavy metal capable of absorbing soft X-ray and visible beam, formed thereon. In order to obtain a good X-ray transmissivity, the thickness of the mask substrate must be about several .mu.m. In an a X-ray mask having such a membrane structure, the mask substrate is required to have a proper tensile strength and a flatness for ensuring a mask pattern arrangement with a high precision and furthermore it is required for the mask substrate that there occurs neither an increase in temperature due to X-ray exposure nor a positional distortion due to deterioration of the material. Moreover, in the system for positioning the X-ray transmission type mask close to a silicon wafer or the like and transferring a mask pattern, the mask substrate must have a high visible beam transmissivity when the transmission is carried out with a visible beam such as a laser beam for highly accurate positioning of the X-ray mask and a material on which the mask pattern is to be transferred.
However, according to the above prior art (Japanese Patent Kokai No. 53-95965), a mask substrate comprising a thin membrane of light element material has been produced by forming a membrane from silicon nitride SiNx having a desired atomic composition only by reducing the internal tensile stress by changing the NH.sub.3 /SiH.sub.4 ratio of a reactant gas in the chemical vapor deposition to increase the effective tensile strength of the membrane per se. However, the X-ray mask comprising such a mask substrate has suffered from the problem that positional distortion occurs due to material deteriorations induced by radiation damage from X-ray exposure as disclosed in "The 1st Microprocess Conference (1988), pages 80-81".